The present invention relates to a process for continuously producing monosilane, the demand for which is recently increasing as a raw material for epitaxy of silicon with a high purity and for amorphous silicon for solar cells.
A known process for producing monosilane is a process for producing monosilane gas by disproportionating a hydrogenated silicon chloride such as trichlorosilane in the presence of a tertiary amine hydrochloride as a catalyst (JP-B-64-3804 and JP-B-63-33422).
Furthermore, another known process is a process for producing monosilane gas by packing a solid catalyst in a reaction column and disproportionating dichlorosilane therein (JP2,648,615). However, since the conversion reaction to monosilane is an equilibrium reaction, the equilibrated conversion ratio has not necessarily been high heretofore, from 10% to 18%, and a large-size apparatus has been required to achieve a desired production amount.
Another known process is a process for continuously producing monosilane readily and efficiently with a large production amount of monosilane from trichlorosilane and dichlorosilane as raw materials (production amount per hour in use of an apparatus with the same reaction performance). EP 2085358 discloses a process for continuously producing monosilane by means of a monosilane production apparatus comprising a reaction column; the process comprising supplying at least one of trichlorosilane and dichlorosilane to a middle stage of the reaction column. A problem of this invention is the large consumption of energy in producing monosilane from chlorosilanes. For continuously producing monosilane, trichlorosilane (TCS) is commonly used as a feed material. In said process, in order to manufacture one mole of monosilane, four moles of TCS feed is necessary. Besides, three moles of silicon tetrachloride (STC) is manufactured as a by-product. To keep plant operation, seamless feed material supply and by-product collection are required; therefore logistics control for feed and by-products materials has been a serious burden for silane plant administrator.
It is necessary to implement a process with a reduction of consumption of energy compared to the process described above.
The present invention aims at solving the problem by removing the inconvenience of the process described above and to improve the reduction of consumption of energy.
The present invention resides in the following aspects.
A process for continuously producing monosilane by means of a monosilane production apparatus comprising a reaction column, at least two upper condensers each of which has a reflux feed pipe serially connected to a top portion of the reaction column, a bottom reboiler of the reaction column, and an evaporation tank connected to a bottom portion of the reaction column; the process comprising:
a) supplying dichlorosilane or a mixture of tetrachlorosilane, trichlorosilane, dichlorosilane, and monochlorosilane where the ratio of the number of total hydrogen atoms to the number of total chlorine atoms H/Cl is in the range from 0.6 to 3 to an upper stage of the reaction column via an upper feed injection point. Preferably, only dichlorosilane is supplied;
b) supplying a catalyst to said upper stage of the reaction column via a lower injection point;
c) introducing a resultant mixture containing monosilane, monochlorosilane, dichlorosilane, and trichlorosilane from the top portion of the reaction column to the plurality of upper condensers;
d) separating monosilane from condensates containing monochlorosilane, dichlorosilane, and trichlorosilane in the upper condensers;
e) recycling the condensates after separating monosilane, through the reflux feed pipes to the upper stage of the reaction column;
f) bringing the condensates into contact with the catalyst in the reaction column; characterized in that a monosilane rich gas, including monosilane as the primal component by molar basis, preferably comprising more than 60% of monosilane, and more preferably comprising more than 80% of monosilane is produced via a pipe connected downstream the upper condensers.
The upper stage of the column means a region higher than 70% of the column in height.
According to one embodiment, said catalyst is at least one of a tertiary aliphatic hydrocarbon-substituted amine and a tertiary aliphatic hydrocarbon-substituted amine hydrochloride.
According to another embodiment, the tertiary aliphatic hydrocarbon-substituted amine and the tertiary aliphatic hydrocarbon-substituted amine hydrochloride are represented by the following formulae (A) and (B), respectively: R1R2R3N (A); R1R2R3NH+Cl− (B), where each of R1, R2 and R3 is an aliphatic hydrocarbon group, the carbon number of each of R1, R2 and R3 is at least 2, and R1, R2 and R3 are the same or different.
According to one embodiment, the process comprises the step:
g) withdrawing a bottom recovery liquid containing tetrachlorosilane, trichlorosilane and the catalyst from the bottom portion of the reaction column, introducing the bottom recovery liquid into the evaporation tank, and recycling the catalyst recovered from the bottom portion of the evaporation tank, to the reaction column.
According to one embodiment, the number of the upper condensers is from 2 to 5.
According to another embodiment, the process comprises after step g), the step:
h) recycling trichlorosilane as feed material toward the reaction column via a tetrachlorosilane/trichlorosilane separation column.
According to another embodiment, the reaction column comprises a mixing tray section between the upper feed injection point and the lower catalyst injection point, preventing liquid catalyst freezing by direct contact with cold reflux liquid. In addition to that, this mixing tray section located over the reaction tray section facilitates vapour-liquid material and heat transfer toward their equilibrium at the top of the reaction column by mixing the reflux liquid and the gaseous mixture formed by disproportionation reactions from the reaction tray section containing monosilane, monochlorosilane, dichlorosilane, and trichlorosilane.
Said catalyst maybe a catalytically active solid matter including, for example, solid substances in which amino groups or alkylene amino groups are carried on a frame of polystyrol cross-linked by divinylbenzol. Amino groups of alkylene amino groups include, for example: dimethylamino, diethylamino, ethylmethylamino, di-n-propylamino, di-iso-propylamino, di-2-chloroethylamino, di-2-chloropropylamino groups and their hydrochlorides or the trialkylammonium groups which are produced therefrom by means of methylation, ethylation, propylation, butylation, hydroxyethylation or benzylation and contain chloride as counterion. Of course, catalytically active solid matter containing other anions, e.g. hydroxide, sulphate, hydrogen sulphate, bicarbonate and others, can be introduced into the process according to the invention in the case of quaternary ammonium salts or protonated ammonium salts. However, conversion into the chloride form will inevitably occur as time passes due to the reaction conditions, even in the case of organic hydroxy groups. Therefore ammonium salts containing chloride as counterion are preferred.
Suitable catalytically active solid matter also includes, for example, solid substances consisting of a frame of polyacrylic acid, especially a frame of polyacrylamide, which has bound trialkylbenzylammonium, e.g. via an alkyl group.
Another group of catalytically active solid matter suitable for the process according to the invention includes, for example, solid substances in which sulphonate groups are bound to a frame of polystyrol cross-linked by divinylbenzol, the cationic companions of the sulphonate groups being tertiary or quaternary ammonium groups.
More preferably, the catalyst to be used is at least one of a tertiary aliphatic hydrocarbon-substituted amine and a tertiary aliphatic hydrocarbon-substituted amine hydrochloride. Compounds represented by formulae R1R2R3N (A) and R1R2R3NH+Cl− (B) are suitably used for the tertiary aliphatic hydrocarbon-substituted amine and the tertiary aliphatic hydrocarbon-substituted amine hydrochloride, respectively.
In the formulae (A) and (B), each of R1, R2 and R3 is an aliphatic hydrocarbon group, the carbon number of each of R1, R2 and R3 is at least 2, and R1, R2 and R3 may be the same or different.
Such kind of liquid amine catalysts have already been investigated and effective operational parameters were summarized in JP-B-64-3804 and JP-B-63-33422. The tertiary aliphatic hydrocarbon-substituted amine may be, for example, tri-n-octylamine, tri-n-butylamine, and so on. In the above formulae (A) and (B), the carbon number of each of the aliphatic hydrocarbon groups is preferably at least 2 and more preferably from 6 to 15. In the present invention, the above-mentioned catalyst is applied and brought into contact with trichlorosilane/dichlorosilane/monochlorosilane, thereby forming dichlorosilane, monochlorosilane and monosilane in accordance with the following disproportionation formulae (1), (2) and (3):2SiHCl3SiCl4+SiH2Cl2  (1)2SiH2Cl2SiHCl3+SiH3Cl  (2)2SiH3ClSiH2Cl2+SiH4  (3)
Among others, it is preferable to use from 98 to 50 mole %, particularly preferably from 98 to 60 mole % tertiary aliphatic hydrocarbon-substituted amine and from 2 to 50 mole %, particularly preferably from 2 to 40 mole % tertiary aliphatic hydrocarbon-substituted amine hydrochloride.
If the rate of the latter is less than 2 mole %, the catalytic activity is low; if the rate exceeds 40 mole %, hydrochloric acid is released during the reactions, whereby reactions below proceed and monosilane is not efficiently produced.SiH4+HCl→SiH3Cl+H2  (4)SiH3Cl+HCl→SiH2Cl2+H2  (5)SiH2Cl2+HC→SiHCl3+H2  (6)SiHCl3+HCl→SiCl4+H2  (7)
The reaction column is one of a distillation column type, and the reaction column suitably used may be a plate column partitioned by sieve trays, bubble cap trays or the like, or a packed column filled with a packing material such as Raschig ring or pall ring. Since the production of monosilane is a liquid phase reaction through the disproportionation reactions, the reaction column is preferably one having a large liquid hold-up capacity.
The reaction temperature is not constant, either because of a temperature distribution in the reaction column, but the reactions are conducted, for example, in a range of from 10° C. to 150° C., preferably from 30° C. to 120° C. If the reaction temperature is lower than 10° C., the reaction temperature could be too low for the disproportionation reactions to substantially proceed. On the other hand, if the temperature exceeds 150° C., thermal decomposition of the catalyst is likely to take place, which is undesirable. Since the reactions are preferably conducted in a boiling state, the gauge pressure is preferably at a level of from 100 kPaG to 1000 kPaG, more preferably from 100 kPaG to 500 kPaG in order to keep the reaction temperature in the above-mentioned range.
The temperature of the bottom portion is controlled by a bottom reboiler, and tetrachlorosilane which need not be returned to the reaction column is preferably selectively recovered from the bottom portion. Therefore, the temperature of the bottom reboiler is preferably from 100° C. to 150° C., more preferably from 90° C. to 120° C.
The mixture formed by the reactions in the top of the reaction column contains chlorosilanes of monochlorosilane, dichlorosilane and trichlorosilane, and monosilane.
According to the present invention, monosilane which is industrially significantly useful is continuously produced readily and efficiently. Moreover, in the silane production process of the present invention, in order to manufacture one mole of monosilane, only two moles of DCS feed are enough. Besides, only one mole of STC is manufactured as a by-product. One advantage is the reduction of the feed flow rate and byproduct silicon tetrachloride flow rate at the same time. Another advantage is the reduction of logistics for feed and by-products materials. Another advantage of the invention is to minimize reactive distillation duty, resulting less utility consumption and smaller equipment.